In a vehicle equipped with an air conditioner, a cooling fan which cools a radiator for cooling engine cooling water and a condenser of the air conditioner is usually provided. The cooling fan requires a large electric current to rotate, and hence when the cooling fan is rotated continuously at a high rotation speed, power consumption and noise levels increase.
Jikkai Hei07-038625, published in Japan in 1995, discloses a technique of controlling the rotation of a cooling fan in accordance with the outside air condition and engine operating condition in order to suppress the power consumption of an air conditioner to a minimum. In this conventional technique, rotation of the cooling fan is controlled on the basis of the discharge pressure of a compressor, the engine rotation speed, and the vehicle speed. Here, the discharge pressure of the compressor is determined from three parameters, namely the outside air temperature, the engine rotation speed, and the vehicle speed.